Pumping units for oil and water wells often include a framework and a large counter-weighted rocker arm and are provided with various types of controls for reciprocating a sucker rod in the well. There are a number of problems with the conventional rocker arm structures among which are the size and weight of the pumping unit and the strain of impact on the sucker rods at the bottom of the downward stroke. A broken pumping string involves a great expense while a crew of men tries to fish the broken string out through the surrounding casing.
Where used herein, the phrase "oil well" may be considered to include water wells, gas wells and the like.
The present invention provides a compact pumping unit machine for reciprocating a sucker rod in an oil well and a hydraulic control unit which applies a shock absorbing means to a sucker rod at the bottom of its downward stroke. The pumping unit itself includes a compact framework and two opposed vertically, slideable carriages connected by a pair of chains. The chains are commonly connected by a pair of sprokets mounted on a common shaft. The front carriage is slideable within channels of the framework and is attached to the sucker rod. The rear carriage is slideable within other channels of the framework and carries a counter balance unit. Vertical reciprocation of the carriages is controlled by the hydraulic unit which lifts the sucker rod string by hydraulic pressure applied through a piston-cylinder combination. At the top of the stroke of the piston, the sucker rod begins its descent by gravity when the hydraulic fluid inflow to the cylinder ceases and a valve opens to allow the cylinder to drain.
A shock absorbing apparatus is incorporated into the drainage path which includes a pair of flow restriction valves, an accumulator tank between the valves and a pressure by-pass around the restriction valve nearest the cylinder in the flow path. The first restriction valve allows flow at a first rate while the downstream valve allows flow at a lower rate. Thus, excess hydraulic fluid will accumulate in the fluid-tight accumulator tank and the back pressure will build until it overcomes the check valve in the pressure by-pass line. The back pressure released through the check valve will slow the piston descent and thereby slow the descent of the sucker rod. The result is a cushioning or shock absorbing of the sucker rod as it nears the bottom and starts its upward stroke.
The present invention will be described with reference to a compact pumping unit which is especially useful in pumping oil. However, the compact pumping unit can also be used in pumping other fluids.